Thin film semiconductor device and its substrate sheet as well as the method for production thereof

ABSTRACT

A thin film semiconductor device and a method for producing it are described. In the thin film layer of semiconductor of the device, a plurality of large size single-crystalline grains of semiconductor are formed in a regulated configuration, and each of single crystalline grains is equipped with one unit of electric circuit having a gate electrode, a source electrode and drain electrode. Such regulated arrangement of large size single-crystalline grains in the semiconductor layer is realized by a process including a step of irradiating the layer of amorphous or polycrystalline semiconductor with energy beam such as excimer laser so that maximum irradiation intensity points and minimum irradiation intensity points are arranged regulatedly. The device can have a high mobility such as about 500 cm 2 /V sec.

A thin film semiconductor device having arrayed configuration ofsemiconductor crystals and a method for producing it.

BACKGROUND OF THE INVENTION

The present invention relates to a thin film semiconductor device and asemiconductor substrate sheet to be used in the semiconductor device aswell as a method for producing them.

As is well known, a thin film semiconductor device or thin filmtransistor (TFT) comprises a substitute, in which a thin film layer ofsemiconductor materials such as silicon is formed on a base layer ofinsulation materials such as non-alkaline glass, or quarts glass. In thethin film layer of semiconductor, a plurality of channel consisting of asource area and a dry area are formed and each of channels is equippedwith a gate electrode. Generally, the thin film layer of semiconductorconsists of amorphous or polycrystalline silicon. However, a TFT using asubstrate comprising a thin layer of amorphous silicon cannot be usedfor a device which requires a high speed operation owing to its extremelow mobility (usually, approximately less than 1 cm²/V sec). Therefore,recently, a substrate comprising a thin film layer of polycrystallinesilicon is used in order to increase the mobility. Nevertheless, even ina case of using such substrate, the improvement of mobility is limitedbecause of such phenomenon at the time of operation as dispersion ofelectron at boundaries between crystal grains, owing to the fact thatthe polycrystalline thin film consists of numerous crystalline grains ofextreme small size.

Thus, it has been tried to obtain a substrate having a thin film layerwhich makes it possible to increase mobility by avoidingdisadvantageousness such as electron dispersion, by means of making thesize of polycrystalline silicon to be large. For instance, it has beentried to obtain a thin film layer having a semiconductor grains of about1 μm size and having a mobility of about 100 cm²/V sec., by annealing alayer of polycrystalline silicon a high temperature furnace. However,the above process has a disadvantage that inexpensive glass sheets suchas sodium glass sheets clot be used and expensive quartz glass sheetswhich can bear high temperature should be used as the process requiresan annealing by extreme high temperature such as over 1000° C. Asubstrate using such extensive materials is not suited for producing adevice of wide size screens in view of costs.

Some other trials has been proposed in order to obtain a thin layerwhich consists of polycrystalline semiconductor of large size grains, bymeans of irradiating a thin film of amorphous or polycrystallinesemiconductor with energy beams such as excimer laser, instead of usinghigh temperature annealing. By this method, it is possible to enlargethe size of a crystal grain, by using inexpensive glass sheets as thebase layer.

Nevertheless, even by the method using irradiation of excimer laser, thesize of obtained crystal grain could not exceed 1 μm and it isinevitable that sizes of grains become uneven. For instance, in thespecification of JPA 2001-127301, there is described a technology forobtaining a thin film layer of polycrystalline semiconductor of largesize crystal grains comprising following steps; that is,polycrystalizing a thin film of amorphous silicon by a meltrecystallization method, for example, using excimer laser irradiation,then depositing a thin layer of amorphous silicon on the recrystallizedlayer and, crystallizing the whole layers by solid phase growing method,thereby growing original polycrystalline gram to large size grains.However, in the above technique, it is suggested that the maximum sizeof obtained crystal grain is about 1000 nm (1 μm) and sizes are uneven(cf. FIGS. 2 to 5 in the above specification).

Furthermore, there is an important problem, which has been neglected inthe technique of semiconductor device comprising polycrystallinesemiconductor that is, the problem of arrangement mode of crystalgrains. In the thin layer of polycrystalline semiconductor produced byprevious techniques, the arrangement mode of crystal grains in thetwo-dimensional direction is utterly random. No trial for making sucharrangement of crystal grains to be a regulated mode has been made. But,randomness of arrangement of crystal grains causes a serousdisadvantage.

That is, it would be needless to say that the numerous units oftransistor circuits formed in a thin film semiconductor device have tobe arranged in a regulated mods such as geometrical arrangement mode.Therefore, when the sizes of crystal grains are uneven and thearrangement thereof are random (not regulated) in a thin film layer, anunit circuit of one transistor is inevitably set in such a modem toextend to a plurality of crystal grains of various sizes and positions(cf. FIG. 6). This would bring such result that the mobility and theelectron transfer mode of each unit circuit are different one another,and this in turn would bring a bad influence to the quality of thedevice. As the result, when characteristics of every unit circuitsdiffers each other, a device cannot but be designed on the whole bybeing based on the low level characteristics. This is an importantproblem to be solved.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a substrate sheetfor thin film semiconductor devices, in which a plurality of large sizesingle crystalline grains of semiconductor are formed in a regulatedarrangement mode such as a matrix-arrayed configuration, thereby makingit possible to use it as the substrate sheet for a thin filmsemiconductor device in which an unit circuit comprising a sourceelectrode, a drain electrode and a gate electrode is formed on each ofcrystal grain.

It is further object of the present invention to provide a thin filmsemiconductor device, which has a high mobility without being influencedby disadvantage such as unevenness of crystal grain size or electrondispersion occurred in crystal grain boundaries, by meat of setting anunit circuit comprising a source electrode, a drain electrode and a gateelectrode on each of crystal gains which are arranged regulatedly insuch mode as a matrix-arrayed configuration in the thin film layer ofsemiconductor.

It is another object of the present invention to provide a process forproducing a substrate sheet for, a thin firm semiconductor devices inwhich a plurality of large size single crystalline grains ofsemiconductor are formed in a regulated arrangement mode such as amatrix-arrayed configuration.

It is another further object of the present invention to provide aprocess for producing a thin film semiconductor device in which an unitcircuit comprising a source electrode, a drain electrode and a gateelectrode are firmed on each of crystal grains which are arrangedregulatedly in such mode as a matrix-arrayed configuration in the thinfilm layer of semiconductor.

Thus, the substrate sheet for thin film semiconductor device of thepresent invention comprises; a base layer of insulation materials, athin film layer of semiconductor formed on the base layer, a pluralityof single-crystalline semiconductor rains formed in the thin film layerof semiconductor and, said plurality of single-crystalline semiconductorgrains being arranged in a regulated configuration such as amatrix-arrayed configuration in the tin film layer of semiconductor.

The film semiconductor device of the present invention comprises; a baselayer of insulation materials, a thin film layer of semiconductor formedon the base layer, a plurality of single-crystalline semiconductorgrains formed in the thin film layer of semiconductor, said plurality ofsingle-crystalline semiconductor grains being arranged in a regulatedconfiguration such as a matrix-arrayed configuration and, each of saidsingle-crystalline semiconductor grains being equipped with an electriccircuit comprising a gate electrode, a source electrode and a drainelectrode.

The method for producing a substrate sheet for thin film semiconductordevices according to the present invention comprises following steps;namely,

-   -   (a) forming a thin film semiconductor layer of        non-single-crystalline semiconductor such as amorphous or        polycrystalline semiconductor on a base layer of insulation        materials and,    -   (b) crystallizing or recrystallizing said non-single-crystalline        semiconductor to produce a plurality of single-crystalline        semiconductor grains by irradiating it with energy beams, said        irradiation being carried out so that irradiated points to which        maximum irradiation intensity is given and irradiated points to        which minimum irradiation intensity is given are arranged in a        regulated configuration such as a matrix-arrayed configuration.

The process for producing a thin film semiconductor device of thepresent invention comprises following steps; namely,

-   -   (a) forming a thin film semiconductor layer of amorphous or        polycrystalline semiconductor on a base layer of insulation        materials,    -   (b) crystallizing or recrystallizing said amorphous or        polycrystalline semiconductor to produce a plurality of        single-crystalline semiconductor grain by irradiating it with        energy beams, said irradiation being carried out so that        irradiated points to which maximum irradiation intensity is        given and irradiated points to which minimum irradiation        intensity is given are arranged in a regulated configuration        such as a matrix-arrayed configuration,    -   (c) forming a gate electrode on each of single-crystalline        grains in the thin film semiconductor layer, which has been        produced by said step (b) and,    -   (d) fabricating an electric circuit in each of said        single-crystalline semiconductor grains by forming a source        electrode and a drain electrode therein.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is pattern diagrams showing steps of one embodiment of theprocess according to the present invention for manufacturing a thin filmsemiconductor device.

FIG. 2 is a pattern diagram for illustrating one embodiment ofdistribution state energy beam intensity in two-dimensional directionsin the irradiation step according to the process of the presentinvention.

FIG. 3 is a pattern diagram illustrating a profile of variation ofenergy beam intensity between a maximum value and a minimum value in theprocess according to the present invention.

FIG. 4 is a pattern diagram illustrating an alignment state of singlecrystalline grains after finishing the irradiation of energy beams inthe process according to the present invention.

FIG. 5 is the pattern diagrams illustrating embodiments of thepositional relationship of electrodes with crystal grams in the thinfilm semiconductor device of the present invention.

FIG. 6 is a pattern diagram illustrating the positional relationship ofelectrodes with crystal grains in the thin film semiconductor devices.

FIG. 7 is a pattern diagram illustrating the pattern of configuration ofmaximum intensity irradiation points and minimum intensity irradiationpoints such as mentioned in FIGS. 2 through 4, with a three-dimensionalmodel pattern.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In the thin film semiconductor device of the present invention, it ispreferred to use a glass sheet hag a strain point not exceeding 700° C.as the material of base layer of substrate sheet. But, it is possible touse various kinds insulation materials other than glass, for example,ceramics or plastic films having appropriate heat resistance.

On the above base layer, a thin film of semiconductor in which singlecrystalline semiconductor grains are arranged in a regulated arrangementmode such as a matrix-arrayed mode is formed. The formation of the thinfilm layer is carried out by depositing a layer of semiconductor ofnon-single crystalline character on the base layer, and then, bychanging the deposited layer to a layer of polycrystalline semiconductorwhich includes large size crystal grains, by means of irradiation ofenergy beams such as er laser beam.

As the semiconductor of non-single crystalline character, amorphoussemiconductor or polycrystalline semiconductor consisting of small sizepolycrystalline grains may be used. By the process according to thepresent invention, any type of the above semiconductor can be changed toa thin film semiconductor of the preset invention, by crystallizing orrecrystallizing them. It is desired that the thickness of thin filmsemiconductor layer is 10 through 200 nm, especially 50 through 150 nm.

Usually, when a semiconductor layer of the above non-single crystallinecharacter is formed on the base layer a thin control layer for adjustingheat-conduction and crystallization-orientation consisting of suchmaterials as silicon oxide, silicon nitride (SiNx) is formed between thebase layer and the semiconductor layer. This layer has such functions asto block the diffusion of impurities such as glass components from thebase layer to the semiconductor layer and to bring the intentionaluniformity of heat distribution of the semiconductor layer forcontrolling the orientation of crystals. The thickness thereof isdesired to be 20 through 1000 nm, especially 200 through 300 nm.

It is also usual to form the second control layer for adjusting heatsconduction and crystallization further on the above non-singlecrystalline semiconductor layer (the first control layer). This secondcontrol layer has a function same as the first control layer, namely, tobring the uniformity of heat distribution and to control the orientationof crystals in the semiconductor layer in the process of crystallizationby irradiation. Materials such as silicon-oxide, silicon-nitride,silicon-oxi-nitride or silicon-carbonate (SiC) can be used therefor. Thethickness of the layer is desired to be 50 through 500 nm, especially100 through 800 nm.

When the above two control layers are formed, the thin filmsemiconductor layer is formed between the two control layers. In thiscase, firstly the material of the first control layer is deposited as athin M1 on the base layer of insulation material, then, the material ofthin film of non-single crystalline semiconductor is deposited on thefirst control layer, and further, the material of the second controllayer is deposited on the above semiconductor layer. Thereafter, theirradiation of energy beams from the upper side is carried out tocrystallize or recrystallize the layer of non-single crystallinematerial.

Now referring to (a) through (d) in FIG. 1, which shows an embodiment ofeach stages of the process according to the present invention, (a) showsthe first stage of deposition of layers, where the first control layer20 for adjusting heat-conduction and crystallization is deposited on thebase layer 10, and non-single crystalline layer 30 of amorphous orpolycrystalline semiconductor is deposited on the first control layer20. Further, second control layer 40 is deposited on the non-singlecrystalline layer 30.

As shown in (b), non-single crystalline layer 30 shown in (a) is changedto a layer consisting of single-crystalline area 50 and non-singlecrystalline area 51, by irradiation of energy beams.

It is of course that (b) shows the sectional pattern view of onesingle-crystalline area and a substrate sheet of the present inventioncomprises of a plurality of such single-crystalline semiconductor area.

Next, as shown in (c), gate electrode 60 is formed on the substratesheet and, source area 70 and drain area 71 are formed in the singlecrystalline semiconductor area 50 by implanting donor impurities 75 suchas phosphorous ions there into, using gate electrode 60 as theimplantation-mask.

Further, as shown in (d), an insulation layer 80 of such material assilicon oxide is deposited on the second control layer 40 and, afterperforating contact-holes through the second control layer 40 and theinsulation layer 80, source electrode 90 and drain electrode 91 areformed by deposition of Aluminum (Al)/Molybdenum (Mo) into thecontact-holes and by patterning it, thereby completing a thin filmsemiconductor device.

In the above step (b), it is preferable to use the excimer laserirradiation as the means for irradiation. However, irradiation meansother than the excimer laser, for example, pulsed argon lasers or YAGlasers, may be used.

In order to obtain a thin film semiconductor layer in which singlecrystalline semiconductor grains are arranged in a regulated alignmentmode such as a matrix-arrayed configuration mode by irradiation ofenergy beams, the irradiation should be carried out in such energyintensity distribution mode as in which the irradiation energy intensitychanges successively in two-dimensional directions between the maximumvalue and the minimum value at every predetermined intervals, andmaximum points and minimum points appear one after another in order. Inother words, the irradiation should be carried out so that irradiatedpoints to which maximum irradiation intensity is given and irradiatedpoints to which minimum irradiation intensity is given are arranged in aregulated configuration such as a matrix-arrayed configuration mode.

For example, as shown in FIGS. 2, 3 and 7, the irradiation is carriedout in the intensity distribution mode in which irradiation energyrepeats such change as “maximum value (Emax)→minimum value(Emin)→maximumvalue(Emax)” two-dimensionally (in the x, y both directions) in arectangle region of 5×5 mm, at every intervals of 10 μm. The abovechange of irradiation energy intensity can be realized by bringing thevariation of the irradiation energy intensity distribution, using aphase shift mask. And, it is desirable that the mode of change betweenthe maximum value and the minimum value is a successive changesubstantially as shown in FIG. 3.

Determination of the degree of the maximum value and the minimum valueto how much value may be based on the film thickness of the noncrystalline semiconductor layer as well as the film thickness and thethermal conductivity of the first and the second control layers. Forexample, the minimum energy intensity may be determined to be anintensity which bogs the temperature which doesn't melt the thin filmsemiconductor during the irradiation period, and the maximum value maybe determined to be an intensity which is necessary and sufficient tomelt the thin film semiconductor during the irradiation period. Amelting threshold level (Emth) should be exited between the maximumvalue (Emax) and the minimum value (Emin), as shown in the FIG. 3.

It is of course that the face shape of the irradiation beam is notlimited to a square shape of 5×5 mm as mentioned above and a be venouspolygon shapes. Further, the arrangement mode of maximum value pointsand minimum value points is not limited to the rectangular lattice modeand may be various shape modes, for example, the mode of delta shapedlattice.

By carrying out the irradiation of energy beams to the thin firmsemiconductor, areas to which the minimum energy intensity (namely, theintensity less than the melting threshold value) is irradiated are notcompletely melted, and therefore, the crystal nucleus are first formedin this areas. Then, crystals become to grow in the two dimensionaldirection towards the ax point, as shown by arrows mentioned in FIG. 4.On the other hand, in areas of maximum energy intensity points where thetemperature becomes highest and in areas which become to be region ofcrystal growth, the formation of minor size or fine size crystals orboundaries of large size crystal are resulted, owing to mutual jammingof growing crystals having different direction of growth. Thus, as theresult, a substrate sheet of thin film semiconductor comprising aplurality of single crystalline semiconductor, whim are formed at areasnearby the melting threshold points and each of which bas a crystal sizeover 4 μm, can be obtained (cf. FIG. 4).

The size of the single crystal can be adjusted by varying the intervalsbetween maximum point of irradiation energy. For instance, when theirradiation of XeCl excimer laser of 308 nm wave length is carried outby making the intervals between maximum intensity point to be 12 μm, asubstrate sheet of thin M semiconductor, in which single crystal grainsof nearly 5 μm size are arranged regularly, can be obtained. It isdesirable that, as the substrate sheet to be used for the thin filmsemiconductor device of the present invention, the size of each singlecrystal grain in the substrate sheet is not less than 2 μm.

Then, on each of single crystalline grains in the thin filmsemiconductor substrate sheet obtained by the process as mentionedabove, the electrode materials such as Molybdenum-Tungsten alloy (MoW)is deposited with, an appropriate thickness (for example, 300 nm),thereby forming a gate electrode. Then, after forming a source area anda drain area respectively using the gate electrode as theimplantation-mask, an isolating-interlayer, with insulation materialssuch as silicon oxide, which covers the gate electrode is formed.Further, after forming contact holes by perforating through the secondcontrol layer at the position above the source area and the drain area,electrode materials such as aluminium/Molybdenum are deposited andpatterned in the contact hole.

Thus, as shown in (a) and (b) of FIG. 5, a thin film semiconductordevice, in which one unit electric circuit in each of single crystals isarranged regulatedly, can be obtained. A thin film semiconductor deviceof this type can have a high mobility (or example, over 300 cm²/V.sec)exceeding the mobility of conventional devices in which a substratesheet comprising polycrystalline semiconductor film is used.

It is possible to omit the setting of electrodes for some of singlecrystals or to set plural units of circuit for one single crystal.Further, though the above explanation is described on the production ofa thin film transistor of N-channel type, it is possible to apply thetechnique of the present invention to a transistor of CMOS type, bymaking partial masking and doping impurities one after another. It isalso possible, instead of using the second control layer as the gateinsulator, to remove the second control layer by etching after formingthe layer of single crystals, and forming a new gate insulation layer atthe removed portions. “Islands separation”, by using etching methodbefore or after the crystallization process, may be carried out, whenthere is a possibility of occurrence of current leakage betweenadjoining transistors.

EXAMPLE

A non-alkali grass sheet, manufactured by Corning Glass Works, with theoutside dimension of 400×500 mm, the thickness of 0.7 m and the strainpoint of 650° C. was prepared as the base layer. On the surface of thebase layer, the first control layer of 200 nm thickness for adjustingheat conduction and crystallization was formed by depositing siliconoxide (SiO₂) with plasma CVD method On the first control layer, a layerof amorphous silicon with 50 nm thickness was deposited, and further,the second control layer of silicon oxide with 200 nm thickness was adeposited an the amorphous silicon layer. These forming of layers by thedeposition of materials were carried out successively in a condition notexposed to the atmosphere.

Next, after annealing and dehydrating the layer of amorphous silicon,crystallized it by the irradiation of pulsed excimer laser beam of 308nm wave length from the upper side.

The irradiation was carried out by using and an unit of excimer laserbeam in which the beam face was so shaped as to a rectangle of 5×5 mmand given with intensity distribution therein by using a phase shiftmask. The mode of intensity distribution was such that 250 thousands ofmaximum value points were arrayed at 10 μm intervals in the form ofsquare-lattice, in the rectangle of 5×5 mm. The melting threshold valuewas approximately 0.5 J/cm², the maximum value and minimum value oflaser beam intensity were 1.8 J/cm² and 0.1 J/cm² respectively.

The excimer laser irradiation by the above mode was carried out towardsthe whole surface to be irradiated, by moving the position ofirradiation stepwise at 5 mm intervals.

After finishing irradiation, the irradiated sample was subjected to beetching treatment by using SECCO etching method and to observation ofthe crystal size and the shape of grain-boundary in the irradiatedlayer. As the result of observation by using an electron microscope, itwas confirmed that a substrate sheet in which grains of single crystalof nearly 4 μm size were arrayed in a matrix lattice form was obtained.

Next, on the second control layer, a layer of Molybdenum-Tungsten alloy(MoW) was deposited with 300 nm thickness by a sputtering method andpatterning it, at the position corresponding to each single crystal, tofrom a gate electrode. Then source areas and drain areas were formed byimplanting phosphorous ions using the gate electrode as the mask, andthe interlayer insulator was formed by deposing silicon oxide by aplasma CVD method. Further, contact holes were perforated in the secondcontrol layer and interlayer insulator and, by forming alumna films incontact holes, completed a thin film semiconductor device. It wasconfirmed that this device shows the average mobility of 496 cm²/V.sec.

1-29. (canceled)
 30. A substrate sheet for thin film semiconductordevice comprising: a base layer of insulation materials, a thin filmlayer of semiconductor formed on the base layer, and a plurality ofsingle-crystalline semiconductor grains formed in the thin film layer ofsemiconductor, said plurality of single-crystalline semiconductor grainsbeing arranged in a matrix-arrayed configuration in the thin film layerof semi-conductor in a mode such that each of the single-crystallinegrains adjoin to the next single-crystalline grains with beinginterposed by boundary areas formed in between the grains and, each ofsaid single-crystalline semiconductor grains having the grain size of atleast 2 μm.
 31. A substrate sheet for thin film semiconductor devices ofclaim 30, wherein a control layer for heat-conduction andcrystallization is formed in between the base layer and the thin filmlayer of semiconductor.
 32. A thin film semiconductor devices of claims30 or 31, wherein a control layer for heat-conduction andcrystallization is formed on the thin film layer of semiconductor.
 33. Athin film semiconductor device comprising: a base layer of insulationmaterials, a thin film layer of semiconductor formed on the base layer,and a plurality of single-crystalline semiconductor grains formed in thethin film layer of semiconductor, said plurality of single-crystallinesemiconductor grains being arranged in a matrix-arrayed configuration ina mode such that each of the single-crystalline grains adjoin to thenext single-crystalline grains with being interposed by boundary areasformed in between the grains, and each of said single-crystallinesemiconductor grains having the grain size of at least 2 μm, and beingequipped with an electric circuit having a gate electrode, a sourceelectrode and a drain electrode.
 34. A thin film semiconductor devicecomprising: a base layer of insulation materials, a thin film layer ofsemiconductor formed on the base layer, and a plurality ofsingle-crystalline semiconductor grains formed in the thin film layer ofsemiconductor, said plurality of single-crystalline semiconductor grainsbeing arranged in a matrix-arrayed configuration in a mode such thateach of the single-crystalline grains adjoin to the nextsingle-crystalline grains with being interposed by boundary areas formedin between the grains, each of said single-crystalline semiconductorgrains having the grain size of at least 2 μm, and electric circuitbeing arranged over the plurality of single-crystalline grains in a modeof CMOS type.